The present invention relates to a light triggered semiconductor device having a light triggered auxiliary thyristor for triggering a main thyristor, and in particular to a light triggered semiconductor device having an auxiliary thyristor disposed outside the package of the main thyristor.
In recent years, light triggered thyristors which can be controlled to be triggered by light signals came into use as thyristors having high blocking voltage and large current capacity applied to high-voltage conversion devices such as high-voltage DC transmission devices.
As such a light triggered thyristor having high blocking voltage and large current capacity, a light triggered thyristor device of the so-called light triggered auxiliary thyristor system is known. In this system, a small capacity light triggered thyristor is used as the auxiliary thyristor and the main thyristor is triggered by the output of the auxiliary thyristor as disclosed in Japanese Patent Unexamined Publication Nos. 55-52261 and 56-56683, for example. Since the light triggered auxiliary thyristor itself may have a small current capacity in this system, the temperature rise at the semiconductor junction of the auxiliary thyristor can be limited to a value lower than that of the main thyristor. Accordingly, the most important development problem relating to the characteristics of the light triggered thyristor, namely the trade-off relationship between light sensitivity and dv/dt capability is easily improved. On the other hand, a new problem is incurred. That is to say, special consideration must be paid to a coupling structure between the light triggered auxiliary thyristor and the electrical triggered thyristor for the main circuit.
That is to say, the light triggered thyristor device disclosed in the above described Japanese Patent Unexamined Publication No. 55-52261 has a light triggered auxiliary thyristor disposed outside the package of the main thyristor and attached to a radiator. In this case, it is difficult to replace the auxiliary thyristor alone. In addition, the above described radiator must have a special structure for fixing the auxiliary thyristor.
On the other hand, the light triggered thyristor disclosed in the above described Japanese Patent Unexamined Publication No. 56-56683 has an auxiliary thyristor contained in the package of the main thyristor. In this case, the above described package must have a portion for fixing the auxiliary thyristor, resulting in the complicated package structure. Further, the auxiliary thyristor might be affected by the heat confined inside of the package.
In the light triggered thyristor device of this light triggered auxiliary thyristor system, it is desired that the combination of blocking voltage, current capacity and turn-on time between the auxiliary thyristor and the main thyristor is provided with sufficient freedom. This problem of freedom is not recognized in the above described prior art.